Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|Development and Culture of Feather Stars|
|Written by Tomoko Shibata, Ph.D.|
Feather stars are a group of crinoids without stalks, which is different from sea lilies (stalked crinoids) that have a stalk throughout their life (fig.1). There are more than 500 species of feather stars in the world and the depths of their habitat range from the littoral zone to several thousands of meters in depth. Taxonomically, crinoids belong to echinoderms together with sea urchins, starfish, brittle stars and sea cucumbers. The body of a feather star shows pentaradial symmetry, which is one of the key characteristics of echinoderms.
Figure 1: An 8 month-old juvenile of Oxycomanthus japonicus.
Feather stars are benthic animals. They usually cling to hard substrates such as corals and rocks with root-like structures called cirri (fig.2). Usually they do not migrate actively, but if they need to, they can crawl with their cirri to change their microhabitat. One of the distinctive structures of a feather star is its arms. A feather star has 10 to 120 arms, and these numbers differ between species and change during various growth stages; five arms extend from the central disc and each branches once or several times. Each arm has pinnules extending from both lateral sides (fig. 3, left).
"Feather stars are a group of crinoids without stalks. There are more than 500 species in the world and the depths of their habitat range from the littoral zone to several thousands of meters in depth."
Arms and pinnules are organs for feeding and locomotion. They have well-developed muscles, ligaments and nerve cords. Feather stars are suspension feeders, which eat planktonic organisms such as diatoms, foraminifera, small crustaceans and young molluscs, as well as organic particles (Rutman and Fishelson 1969, Kitazawa et al. 2007). Most food which feather stars ingest is less than 400 μm in size (Rutman and Fishelson 1969). A feather star captures food by tube feet distributed along its pinnules (fig. 3, middle). Mucus secreted from the tube feet serves as a trap for food (Nichols 1960). Food grooves run along the surface of pinnules and arms, and finally lead to the mouth located in the central part of the body (fig. 3, right). Captured food is transported through these grooves to the mouth. The digestive tube is located in the central body and the anus opens closely to the mouth (fig. 3, right).
Figure 2: Feather stars anchor themselves to rocky substrate by moveable appendages called cirri.
"Feather stars are suspension feeders, which eat planktonic organisms such as diatoms, foraminifera, small crustaceans, young molluscs and organic particles."
A feather star has calciferous endoskeleton consisting of small plates, and each plate is connected by short muscles and ligaments. Such structures provide ample mobility and flexibility to the animal.
Figure 3: Different anatomical structures of a feather star. Left: an arm with elongated pinnules. Middle: enlarged view of an arm. A food groove runs along the arm and pinnules. Fine tube feet are distributed on the pinnules. These pinnules greatly increase arm surface area, which allows for effective capture of plankton by tube feet. Right: Upside view of the central disc of a feather star. Food grooves converge at the mouth. The mouth and anus are located on the central disk, in close proximity (photographs: Fumiaki Sodeyama and Dr. Tomoko Shibata).
Reproduction and Development
Most feather stars are dioecious, which means that separate male and female animals exist. A gonad develops in each pinnule, which is located in a relatively proximal part of the arm. They eject eggs or sperm from gonads and fertilization occurs in the seawater. There are some variations in spawning style of females according to species. In some species, a female ejects all of its eggs simultaneously. In other species, a female ejects eggs multiple times in small volumes. After spawning, some specimens wave their arms to disperse the eggs, and some leave eggs attached onto the pinnules (Holland 1991).
Figure 4: Spawning of a female O. japonicus. Eggs are ejected from each gonad, located in each pinnule. After spawning, the female waves its arms and disperses the eggs into seawater.
"Most feather stars are dioecious, which means separate male and female animals exist."
Regarding Oxycomanthus japonicus, a well-studied species in Japan, spawning occurs once a year, on the evening of the neap tide in the second half of October. Spawning of O. japonicus is shown in figure 4. Comparison between the spawning dates from 2000-2006 and those from 1937-1955 revealed that spawning had shifted one neap-tide cycle later over these past 60 years. This phenomenon could have been due to a rise in average seawater temperature, as this influences the rate of sexual maturation and the timing of spawning. Because no data on seawater temperatures from 1937-1955 were available, our team compared data from 2000-2006 and those from 1964-1973, the oldest period for which records were available. The analysis showed that the average seawater temperatures in September and October were significantly increased over these 40 years (fig.5).
Figure 5: Comparison of monthly seawater temperatures (1 m in depth) between the period from 1964-1973 (blue line) and that from 2000-2006 (red line). Average seawater temperatures in March, May, September and October for 2000-2006 were significantly higher than those for 1964-1973 (arrows, p<0.01; T-test).
It is empirically known that the animals tend to spawn after the seawater temperature decreases below 22°C (Kubota, 1988), and it is possible that an increase of seawater temperature caused this delay in spawning (Shibata et al. 2008).
Recently, the existence of a hermaphroditic feather star Dorometra sesokonis was reported (Obuchi et al. 2008). In this species, an adult has both ovaries and testes. This report demonstrates a new variety of reproduction mode in feather stars.
Regarding O. japonicus, about 16 hours after fertilization, larvae hatch from eggs and start swimming in the seawater. The body shape of a swimming larva, called doliolaria, is like a barrel (fig.6, left). The doliolarian stage lasts for a few days, after which they settle onto a substrate and metamorphose from a bilateral body to pentaradial body.
Figure 6: Developmental steps of feather stars. Left: doliolariae. Middle: cystidean after settlement and metamorphosis. Right: pentacrinoid with opened mouth.
At this stage, they are called cystideans (fig.6, middle). About one week after settlement, they open their mouth at the top of their body and start feeding. From this moment, they are called pentacrinoids (fig.6, right). Several days after opening the mouth, they grow 10 arms (fig.7, left). That is, five arms elongate from the central disc (called centrodorsal) and each branches once. They display a sessile stage for about two months, then detach the stalks and start free-living (fig.7, right).
Figure 7. Left: Pentacrinoid with 10 arms. Right: Juvenile after detachment of its stalk.
In many species, the number of ten arms remains stable throughout life. However, in some species, arm number increases in the developmental process. In O. japonicus, 6 months after fertilization, this increase starts. They spontaneously cut off an original arm at the proximal point (close to the central disk) and discard it. After this, two arms grow from that point. In this way, one original arm is replaced with two new arms (fig.8, left). This is called branching regeneration. It does not occur simultaneously in all arms, but rather one after another (fig.8, right, Shibata and Oji 2003). In several months, cutting off and branching regeneration occurs in 10 original arms, and as a result the number of arms becomes 20. In these 20 new arms, the same phenomenon repeats itself, yielding an adult with a final number of 40 arms.
The individuals in the bay culture system (explained below) reach sexual maturation one or two years after fertilization. Maturation time depends on the population density of individuals. To let them mature early, this density should be low so that they can obtain enough food from the water.
Figure 8: Branching regeneration. Left: location where self-cutting of an original arm and regeneration of two new arms occurs. Right: An illustration of the chronological process of arm number increase in an individual.
Culture in the Bay
Long-tem culture from fertilization to sexual maturation in an aquarium is difficult because of lack of nutrition, and it has not been accomplished yet. Instead, a culture system in the bay has been established. The culture method we adapted is as follows: Fertilized eggs were incubated in a plastic container 30 cm in diameter, with about 7,000 eggs in ten liters (about 2,6 USG) of seawater (fig.9, left). The container was covered with plastic film to prevent water from evaporation and dust contamination. The seawater in the culture container was replaced with freshly filtered seawater every other day. Room temperature was (and should be) kept close to seawater temperature in the natural environment. Seven days after fertilization, most larvae settle onto the wall or bottom of the container and metamorphose into stalked juveniles called cystideans. At this stage, they should not be removed from the container by any disturbance, so the container was transferred to the sea. The container was put in a net bag of 1 cm mesh, and the opening of the bag was tied and hung from a floating device anchored in the bay (fig.9, right). The important point of suspending the container is that the open sides should be placed downwards. This avoids the accumulation of sediment in the container, which causes mortality of the settled juveniles on the bottom.
Figure 9: Equipment for mariculture in the bay. Left: Culture of swimming stage larvae in containers. Right: containers put into net bags and suspended in the ocean.
A major problem of the bay culture system is the appearance of sessile, encrusting organisms that may attach to the container and the net. During a warm season, there are many organisms such as tunicates, barnacles, sea anemones, clamworms, mussels and sea mosses both inside and outside of the container. They interfere with feather star feeding, so removal of these animals is required once every one or two months. However, complete cleaning is not recommended because after detachment from their stalks, free living stage feather stars require something to cling to. When cleaning the containers, we typically remove tunicates and mussels, but leave shells of barnacles and clamworm tubes after breaking. In general, we keep the container walls rough. This method was originally developed by Grimmer et al. (1984) and improved by Shibata et al. (2008).
"The next challenge is to culture feather stars in a closed system up to sexual maturation and observe their whole life cycle in the aquarium."
Challenge of Culture in a Closed System
We are able to keep feather stars in an aquarium, but it is difficult to rear them to larger sizes. The problem is the lack of food. As mentioned above, they feed on plankton and organic particles in their natural environment. Also, they seem to feed for many hours a day. In the aquarium, providing them with enough food would cause water pollution, so feeding them sufficiently is not easy. Providing them with live feeds such as brine shrimp (Artemia salina) would be a solution. We try to feed them ground copepods, wheel animalcules (rotifers such as Brachionus plicatilis), diatoms and brine shrimp as much as possible. The next challenge is to culture feather stars in a closed system up to sexual maturation and observe their complete life cycle in the aquarium.
Figure 10: A bright-yellow feather star on a reef in the Philippines. Current aquaculture techniques do not yet allow for successful husbandry and breeding of these unique creatures in closed systems (photograph: Hans Leijnse).
All photographs, unless otherwise stated, by Dr. Tomoko F. Shibata.
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